The survey of maternal and fetal effects on calving time of single pregnancy in Holstein dairy cattle Talebkhan Garoussi. M., Fallah Rad. A. H., Mish Mast, Z. Theriogenology Section, Department of Clinical Sciences, Faculty of Veterinary Medicine, Ferdowsi University of Mashhad, P. O. Box: 917751793. Mashhad-Iran E.mail: [email protected] Tel & Fax: 0098 511 8763852 Introduction Parturition is one of the most important aspects of cow's biology due to the vital role in the milk and calf production. The aim of this study was to find out the correlation between the calving time related to the maternal and fetal factors such as: number of parity, pregnancy duration, calf sex and live weight, rate of still birth, retention of fetal Membrane (RFM) and finally the effect of birth of season in Holstein dairy cows. Materials & Methods During a 4 years study, the time of 2135 single calving were evaluated. The maternity section was stress free in the herd. The time of parturition stratified into 2 groups, 1day time from 6 AM to 6 PM and 2-night time from 6 PM to 6 AM. The above factors mentioned were evaluated. Results were analyzed using Chi-square statistical method. Results It was showed that 53.8% and 46.2% cows calved during the day and night time, respectively. One hour period from 08.00-09.00 had the highest deliveries with 119 birth, and at 23.00 had the minimum parturition with 62 births. The rate of RFM during the night and day time was 54.7% and 45.3%, respectively (P<0.05). Most (69.7%) of the still births were occurred during the night (P<0.05). Autumn with 688 calving (32.2%) and spring with 392 births (18.4%) gad the highest and lowest rate, respectively. Discussion & conclusion With preparing silent and stress free environment in calving time, the negative aspects of birth can be reduced and most of the calving will be occurred during the day time.